Delta-Sigma ADC reading wrong value

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In a few designs I have noticed a glitch within the ADC cause it to return a weird and off scale values in certain low voltage ranges (0.08 to 0.09 volts in this case, there is another range a bit higher up). This glitch will affect all ADC readings with the SAR ADC if one input is in this range.

Edit: This happens on the CY8C5888CTI-LP097 / CY8CKIT-059.

Today I have taken the time to find and map those certain voltages only with the Delta Sigma ADC. The type of reference does change the glitch a bit but still within the same voltage range. I will try to get a 16-bit voltage source hooked up to try and get the other range where this happens (It is much smaller).

Testing Steps:

For a simple strategy of testing this phenomena, I interconnected an 8-bit DAC to a 16-bit Delta Sigma converter. Although this does happen with an external input.

The ADC data and control data for the DAC were sent over USB using the CDC driver.

The DAC is setup as 0-1.020 volts, low speed mode.

The ADC is setup as Vssa-2.048, Internal VREF 1.024, bypassed buffer. I have mapped the other buffers and one of a different reference.

Attached is raw CSV file (Voltage vs Time), source code, and an images of the graphed data.

Edit: Added data for a 2 channel SAR with a 14Bit Signal fed into the channels. It looks like there is a possible bit error. This also shows how it affects the whole device.

Thanks, can't wait to hear your feedback.

Nick

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1 Solution

USBUART is a USB CDC device simulating a COM port. ( 64-byte packet exchange)

This is not a UART and requires a different approach to the organization of exchange with the device.

this was discussed a bit here:  PSoC 5LP WaveDAC + ADC_Delsig

View solution in original post

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5 Replies
Vasanth
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250 sign-ins 500 solutions authored First question asked

Hello Nick,

Some clarification about the description . Are you telling that the error is there with both SAR  ADC and Delta sigma ADC ? Did you observe the error before transferring the results over USB?

Best Regards,
Vasanth

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Greetings Vasanth,

In correction to the title, the SAR observation was added after. Yes it

does happen to both.

As for the before USB transfer I would have to get back to you on that

tonight. I do believe it happens still because of weird stuff happening in

one of my systems.

Thanks.

Nick

On Tue, Apr 16, 2019, 8:06 AM VasanthR_91 <community-manager@cypress.com>

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Vasanth
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250 sign-ins 500 solutions authored First question asked

Hello Nick,

Did you confirm whether it is a issue while transfer? Have you been able to see result register on debug mode ?

Best Regards,
Vasanth

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Greetings Vasanth,

I kind of forgot about this as the system works fine if I don't try to debug it through USB.

Yes, I had a look at it this morning and it appears to be a hardware timing issue for the communication. It doesn't cause the result register value to act weird.

Thanks.

Nick

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USBUART is a USB CDC device simulating a COM port. ( 64-byte packet exchange)

This is not a UART and requires a different approach to the organization of exchange with the device.

this was discussed a bit here:  PSoC 5LP WaveDAC + ADC_Delsig

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